[1] http://www.wolframalpha.com/input/?i=32.22+km^3+%2F+296*10^6...
From Wikipedia on Hoover Dam: "Filling of Lake Mead began February 1, 1935, ... ... In the latter half of 1936, water levels in Lake Mead were high enough to permit power generation ..."
To empty Hoover Dam once every 60/80 years doesn't sound absurd to me.
--Love, the Western US.
A sadder thing is that while the Marinas that are now on dry land have a chance to do some upgrades/maintenance, they have no cash since they are very day to day based on the cash inflow from slip fees. That is a tragedy I think, you could completely rebuild the piers and develop a much more durable infrastructure for future use I would expect.